Patent GB-1439874-A (GB)
Title: Apparatus for measuring group delay and attenuation in quadripole
IPC: H04B3/46
Filed: 1973-09-06
Abstract: 1439874 Measuring group delay; measuring phase difference SOC ITALIANA TELECOMUNICAZIONI SIEMENS SpA 6 Sept 1973 [31 Oct 1972] 42023/73 Heading G1U [ Also in Division H3] Group delay and relative attenuation at two different carrier frequencies in a four-pole network is measured by transmitting therethrough a signal having alternate periods of carrier frequencies, f M , f R  commutated at a frequency 2f c  and amplitude modulated at a frequency f m , a further modulation at a frequency f r  being provided during the last period of the reference frequency transmission f R . The group delay is determined from the phase values of the modulating signal in each carrier frequency period, the difference between these phase values actually being measured to provide the difference between the group delays at the reference and measurement frequencies. Signals received from the four-pole network via repeater TL Fig. 3b, are subjected to logarithmic compression LG full-wave rectification RT and amplitude demodulation by a low pass Filter Fl. Phase difference measurement, MR, Fig. 3b A filter F3 selects the modulation signal f m  which  is squared-up ZCD and fed x to a phase comparator RF, whose reference input y is also at f m , and has a phase mid-way between the two phase values of the input x. The comparator RF provides two pulse trains a, r having pulse widths corresponding to the time difference between the signals x, y (i.e. phase difference) and where a represents y lagging x and r represents x lagging y. Thus alternate trains of pulses are emitted from a and r, and these are combined (one being invested) and fed to an averaging filter F 5 . The average of this signal is used to control a crystal oscillator VCXO of nominal frequency fm, whose output is mixed MX with a local signal of 24,000 f m . The sideband at 240 f m  is selected F 6 , squared CV and divided down to f m , Dl, D3, D4. This divided output forms the reference signal y. Thus any deviation of y from the average value produces an output from F 5  which adjusts the loop frequency to correct the error. The signals a, r are fed to a coincidence circuit S where they are gated by signals d 7  occurring once during each carrier period into amplifier AD1 where they are combined as before. Filter F7 tuned to f c  provides a sine-wave output whose amplitude represents the phase difference. This is sampled, converted to frequency and displayed TC1, VC0, DF and also fed to recording apparatus aY. Attenuation measurement, MA, Fig. 3b. Filter F2 is tuned to f c  and thus provides a sinewave whose amplitude represents amplitude difference in the modulating signal in the two carrier frequency periods. Since SIR involves logarithmic amplification, the sine-wave amplitude thus represents the attenuation ratio for the two frequencies. This amplitude is sampled TC2, indicated M1 and also fed to recording apparatus aY<SP>1</SP>. Carrier frequency measurement, UMF, Fig. 3a. In the analogue measurement, PX, the output from rectifier RT (Fig. 3b) is squared SQ and a monostable M provides equal length pulses therefrom. The average level in each transmission period thus represents the carrier frequency. A filter F8 tuned to f c  provides a sine-wave whose amplitude represents the frequency difference. The magnitude is sampled and stored TC3 and fed to recording apparatus aX. The frequency is also measured digitally, MF (see Division H3). Timing pulses K, Z Fig. 3b. Reset pulses K are derived SR from the additional modulation occupying the last 24 msec. of the reference frequency transmission. An inherent delay of 30 msec. places the pulses K at the start of each measurement frequency period. Sampling pulses Z are produced SC during both transmission periods and are derived from signals at frequency f c  and 2f c  in logic RL2 Fig. 6 (not shown).